Wrought nickel base alloy



Sept. 16, 1969 J. F. BARKER 3,467,516

WROUGHT NICKEL BASE ALLOY Filed May 2, 1966 United States Patent O Fcice3,467,516 WROUGHT NICKEL BASE ALLOY James F. Barker, Cincinnati, Ohio,assignor to General Electric Company, a corporation of New York FiledMay 2, 1966, Ser. No. 546,647 Int. Cl. C22c 19/00 U.S. Cl. 75-171 3Claims ABSTRACT F THE DISCLOSURE A wrought nickel ibase alloy havinggood strength properties up to about 1800 F. is provided with improvedweldability in sheet form through a careful balance of the precipitationstrengthening elements Al and Ti and the solution strengthening elementsMo and W in the presence of carbon to form the desired carbides alongwith desired intermetallic compound structure. The element Cr isincluded at a level sufficiently low to stabilize the allow structureand Co has been included to contribute toward hot workability.

This invention relates to nickel lbase alloys and, more particularly, toan improved fabricable nickel base sheet alloy having improvedweldability and resistance to subsequent weld cracking along with a goodbalance of stable tensile and stress rupture properties between roomtem- .perature and about 1800 F.

The invention described and claimed in the United States patentapplication herein resulted from 'work done under United StatesGovernment contract FA-SS-64-1. The United States Government has anirrevocable, nonexclusive license under said application to practice andhave practiced the invention claimed herein, including the unlimitedright to sublicense others to practice and have practiced the claimedinvention for any purpose whatsover.

The advance of technology in power producing apparatus, for example gasturbine engines, continuously identifies new needs for improvedmaterials. One type of alloy more widely used in such apparatus hasbecome known as the nickel base supperalloy because of its general highstrength and oxidation resistance at elevated temperatures.

Tensile strength and stress rupture strength of nickel base super alloysalong with their associated ductilities are important considerations fordesigners in the development of components. However, a very seriousweldability problem can exist when the alloy form is that of sheetmaterial from which components are fabricated. Along with weldability isthe problem of the capability of the material to be heat :treated afterwelding as well as to be repair welded `without cracking.

One emphasis in the development of nickel base super alloys has been onhigh tensile strength and high stress rupture strength. It is nowrecognized :that a practical combination of these strengths along withinherent oxidation resistance and a solution to the welding problem is apractical answer to the use of improved nickel base 3,467,516 PatentedSept. 16, 1969 superalloys in components for advanced power productionapparatus.

Therefore a principal object of this invention is to provide a nickelbase alloy which can be reduced to sheet form and which has an improvedbalance of tensile and stress rupture properties between roomtemperature and about 1800 F. along with the capability of being welded,heat treated after welding and repair `welded Without detrimentalcracking.

This and other objects and advantages will be more readily recognizedfrom the following detailed description and the drawing which is agraphical comparison between stress rupture properties of the alloy4 ofthe present invention and similar known alloys.

It has been found that the above object can be realized from a nickelbase alloy composition consisting essentially of, by weight, 0.07-0.17%C; 0.005-0.02% B; 13-16% Co; l2-15% Cr; 5.5-7% Mo; 3.5-4% A1; 2-2.7% Ti,with the sum of Al and Ti being 5.5-6.5%; 1.4-3.5 W with the balancenickel and incidental impurities.

In general two major Vstrengthening mechanisms are considered withnickel base super alloys. One of these involves the solid solutionstrengthening of the alloy through the use of such elements as tungstenand molybdenum. The other is the precipitation strengthening mechanisminvolving the use of such elements as titanium, aluminum, columbium,etc. in the presence of carbon in the nickel ibase to precipitatevarious carbides or various complex phrases. The present inventionrecognizes a critical relationship and effect between the individualsolution strengthening elements and the individual precipitationhardening elements as separate groups as well as complex balance andinterrelationship between these two groups of elements.

Although tensile properties and stress rupture properties are importantconsiderations, a practical alloy cannot be designed solely with theseproperties in mind if usable structures are to be made from them. Thisis particularly true when the alloys are to be reduced to sheet form forsubsequent fabrication by welding into a structure. Thus ductility,weldability and repair weldability can be equally importantconsiderations if the alloy is to be useful in such wrought form.

As will be shown in the following tables, combinations of solutionstrengthening and precipitation hardening elements can be added to aNi-Cr-Co base material in different proportions to result in unusuallyhigh .tensile and stress rupture strengths. Nevertheless, the alloy ofthe present invention within the .much broader scope of the alloys ofthis type dene an improved alloy which has the unusual combination ofgood tensile and stress rupture properties along with good ductility,fabricability and weldability in an inherently oxidation resistant andstable alloy.

Typical of the alloy forms within the scope of the present invention arethose, the compositions of which are shown in the following Table I.

TABLE I.-WEIGHT PERCENT, BALANCE Ni AND LNCIDENTAL IMPURITIES Thesealloy forms as well as those other alloys the compositions of which areshown in the following Tables IV and VI-I were vacuum induction meltedin approximately pound heats and were cast into 1/2 X 3" x 6 slab ingotsexcept for alloy forms 3 and 4 of Table I. The ingots were conditionedon all sides by grinding and converting to .0S-.06" thick sheet by hotrolling. Alloy form 3 was a 100 pound vacuum induction melted heat andalloy form 4 was a 2400 pound vacuum melted heat.

After reduction to sheet, the various alloy compositions in Table I aswell as those other alloys shown in the various tables which follow wereevaluated first for solution heat treatment to find the gamma primephase solution temperature. With the exception of the alloys outside thescope of this invention including aluminum and titanium at higherlevels, the gamma prime solution temperature was found to lie within therange of 2000-2150 F. and primarily 2050-2l50 F. Therefore, such rangewas selected as the solution heat treatment temperature for the hotrolled sheet material specimens prior to property evaluation. Followingthe solution heat treatment was a typical aging treatment for nickelbase super alloys. In this case it was selected to be at about 1400 F.for a normal period of time such as 16 hours. The following Table IIlists the tensile and stress rupture properties of the alloy forms ofTable I both at room and elevated 3 templ'atlll's aS ShOWIl.

TABLE IL STRENGTH PROPERTIES used in the fabrication of nickel base andother alloys and by electron beam (EB) welding. The following Table IIIsummarizes the results of the weld restraint patch test conducted on thealloys of Table I.

TABLE HL-wE-LD RESTRAINT PATCH TEST STATUS AFTER WELDING, IN CONDITIONINDICATED Type Stress Repair weld relief Age weld Alloy form:

1 TIG No cracks... No cracks--. No cracks. 2 TIG do do Do.

"In the above Table HI the annealing heat treatment was 2025* F. for 10minutes, furnace cool to 1950* F. in four hours then water quench priorto welding into the fixture. The stress relief treatment wasaccomplished by inserting the patch test assembly into a furnacepreheated 3 hours at 2100 F. It was held there for 10 minutes and thenfurnace cooled at the rate of 30 F. per minute to 1200" F. Aging wasperformed at 1400 F. for 16 hours after which it was air cooled. As canbe seen from the above table, the alloys of the present invention passsuccessfully this critical weldability evaluation.

Contrasted with the alloy of the present invention are a number of otheralloys some of which have either tensile or stress rupture strengths orboth slightly better than some forms of the present invention. However,none of these other alloys tested successfully passed the weld restraintpatch test. Thus such other alloys outside the scope of the presentinvention do not provide the unusual combination of good strengthproperties combined with weldability so important to the use of an alloyin sheet form.

Stress rupture 1,400 F. tensile UTS (K si.) 0.2 YS (K si.)

Alloly form:

As was emphasized before, even though the alloy forms of Table I shownin Table Il a good balance of strength properties an equally importantproperty is the ability of a sheet alloy to be welded, stress relieved,aged after stress relief and repair welded so that practical use can bemade of the alloy.

The weldability evaluation used in connection with the alloy of thepresent invention involved welding to a strong retaining back member -a31/2 diameter first disc of 0.05-0.06" sheet having a 1%." diametercentral opening. In this case the back member was a 1/2 thick squareplate each side of which was 6%" long and including a 3 diameter centralopening. After welding the first disc of test material at its outerperiphery symmetrically over the central opening in the restraining backmember, a central circular 11/2 diameter patch or second disc of thesame test material was welded in the 111/2 diameter opening of the firstdisc.

The welding stresses produced in this configuration have been found tobe equal to the yield strength of the test material and simulate theconditions found in a welded fabrication. The patch test assembly isthen given the prescribed heat treatment for the alloy and later arepair test weld is placed radially inwardly from but adjacent the 1'1/2diameter weld around the central plug.

This procedure was used in the evaluation of the alloys discussed here.Welding was accomplished by the well known tungsten inert gas (TIG)welding method widely 1,800 F./11 K s.i.

Percent el. Time (hrs.) Percent el.

TABLE IV.-0THER ALLOYS [Weight percent, balance Ni and incidentalimpurities] Cr Co M0 W Al Tl .AH-Tl TABLE V between the solutionstrengthening elements molybdenum and tungsten which are shown to haveindividual effects rather than to be interchangeable even on an atomicbasis. A third aspect is the combined efrect of these two strengtheningmechanism in various range.

Because of the intentional inclusion of carbon in the [StrengthProperties after 2000-2150 F. solution and 1400 F. age] 1,400 F. tensile1,800 F. stress rupture 0. 2 UTS YS Percent Time Percent (K s.i (K s.iel. (hrs.) K s.i el.

In Table V, UTS means ultimate tensile strength, 0.2YS means 0.2% yieldstrength, percent el. means percent elongation and K s.i. meansthousands of pounds per square inc In Table IV, with the exception ofExamples 14 and 15, the chromium level was maintained within the rangeof 12-15 in order to enhance stability. Alloys such as those evaluatedhere and to which the present invention relates would be expected to beunstable with chromium amounts in excess of about 15 weight percent andparticularly as that percentage approaches However, if the chromiumcontent is below about 12%, for example about 10% shown in Example 14,severe oxidation results at temperatures of about 1800 F. This was thecase with Example 14 in the stress rupture testing. Also comparingExamples 15 and 21 shows the effect of lower chromium in similar alloys.

The other element which was maintained in the alloys of Table IV withinthe range of the present invention is boron. Much has been reportedabout the effect of boron on good stress rupture properties. Therefore,the alloy of the present invention includes boron in the range of atleast 0.005% because less than that amount has little effect on suchproperties. However, the inclusion of boron at above about 0.02% tendsto cause incipient melting of the alloy and hot shortness or poorductility when being worked at elevated temperatures.

The element cobalt has been included in the alloy of the presentinvention in the range of about 13-16% for its benefit on hotworkability. Some of the alloys of Table IV, all of which are outsidethe scope of the present invention, included cobalt within the range ofthe present invention for comparison purposes. In addition, cobaltlevels between 10 and 25% are included to show that increased amounts ofcobalt outside the present invention range at higher levels assist inimproved stress rupture life but at the expense of much lower tensileductility.

Several very important aspects of the alloy of the present invention areemphasized in the compositions of Table IV compared with the strengthproperties shown in Table V. One is the interrelationship oftheprecipitation strengthening'elements aluminum and titanium, the sum ofwhich according to the present invention are limited to the range ofabout 5.5-6.5. Another is the interrelationship range of 0.07-0.17%carbides which are intentionally formed in the alloy of the presentinvention are specifically controlled. Carbon is purposely included at alevel greater than 0.5 weight percent because at that level and belowcarbon functions primarily as a deoxidizer.

With insufficient additional carbon, a continuous carbide film canprecipitate in the grain boundaries. On aging, this film would remaincontinuous and tend to embrittle the alloy. However, at levels of fromabout 0.07 to about 0.17%, there are formed discrete, agglomerated grainboundary carbides which destroy the continuity of the embrittlingcarbide films. This is particularly effective in the range of0.07-0.12%.

To much carbon in the composition will not allow suficient grain growthunless increased solutioning temperatures are employed. However,solutioning temperatures greater than about 2150 F., used in theevaluation of this invention, have been shown to approach the incipientmelting point of the lower melting alloy constituents.

The carbon range of the present invention has been selected specificallyto provide more than required for deoxidization but less than thatamount which will inhibit grain growth. Such range results in theformation of agglomerated carbides while at the same time allowing theuse of practical solutioning temperatures well below the incipientmelting point of any of the lower melting alloy constituents in order tomaintain good properties.

One of the problems in providing a wrought material which can be madeinto sheet form is that its aluminum and titanium content besufiiciently low for workability purposes. The alloys of Examples 9 and10 including the sum of aluminum and titanium at 7.7 and 7.9 totalweight percent respectively could not be converted into sheet because ofcracking during reduction.

The alloys ofv Examples 24 and 26 could be reduced to sheet form becauseof the significantly greater amount of cobalt included in the alloycomposition. However, the tensile properties of the resulting alloy islow, particularly the tensile ductility.

Example 8 which has slightly less of a total of aluminum and titaniumcould be reduced to sheet form but was very brittle as shown-by thetensile ductility of only 1% elongation and the substantially sameultimate tensile strength and 0.2% yield strength.

At the other end of spectrum of alloys the compositions of which areshown in Table IV is the alloy of Example 7 which is within the scope ofthe present invention except for the sum of the aluminum and titaniumcontent and the carbon content. Referring to Table V, it is seen thatExample 7 has good tensile and stress rupture properties. Nevertheless,as will be shown in the following Table VI, alloy 7 cracked in thestress relief step after welding and after being solution heat treatedat 2l00 F. for 1/2 hour followed by air cooling. The mild nature of thiscracking indicates the sum of the Al and Ti contents were slightly inexcess of that desirable for the best weldability.

TABLE VI.-WELD RESTRAINT PATCH TEST 1 EB welded. 2 Severe transversecracking on weldlng.

This reaction after welding of alloy 7 points out a criticalrelationship which exists between the total content of the precipitationhardening elements aluminum and titanium and carbon in the alloy of thepresent invention.

The precipitation hardening elements aluminum and titanium in quantitieslower than that of the present invention are also shown'in Table IV.lFor example, alloy 18 includes a lower total amount of aluminum andtitanium. As a result, the tensile properties are somewhat reduced andthe stress rupture strength is significantly reduced. Other variationsin aluminum and titanium content are shown in Table IV and thecorresponding strength properties are shown in Table V.

Only those alloy compositions shown in Table IV having reasonably goodstrength properties were tested in the weld restraint patch test. Of thealloys tested, including alloy number 6 which is at the lower range ofthe sum after Welding and a severe crack upon aging. Thus it was notsuitable for fabrication into a useful article.

The compositions shown in Table 1V were varied, in addition, to show theeffect of the solution strengthening elements molybdenum and tungsten onthe alloy of the present invention and to show that such elements arenot interchangeable even on an atomic basis. The alloy of the presentinvention includes molybdenum and tungtens in the range of 5.5-7 weightpercent molybdenum and 1.4-3.5 weight percent tungsten. For example, acom parison should be made between Example 7, including 5.5% Mo and 3.2%WWith Example 1l, including 3.4% Mo and 5.4% W with the same total ofprecipitation hardening elements aluminum and titanium. Table VI showsthat Example ll could not even be welded because of severe transversecracking on welding whereas Example 7 was at least weldable even thoughit cracked on stress relief.

'I'he elements molybdenum and tungsten are not interchangeable nor canthey be substituted one for the other in the alloy of the presentinvention. The inclusion of amounts of tungsten greater than 3.5 weightpercent results in poor weldability and severe cracking. Below 3.5% Wthe element can participate properly in solid solution strengthening.Example 8 which included as much as 10.1% W assisted the large amount ofprecipitation strengthening elements in providing a brittle alloy ofvery low stress rupture strength. Below about 1.4%, insutiicienttungsten is present to participate with Mo in the solution strengtheningmechanism. In the absence of tungsten, as shown by AExample 2l,relatively good tensile properties can result but with unusually lowstress rupture life, even with the proper range of the precipitationhardeners aluminum and titanium. Thus it is seen that the elementsmolybdenum and tungsten are critical with regard to the weldability ofthe alloy and play their own individual roles in providing solutionstrengthening along with good weldability.

From time to time, other strengthening elements have been considered foraddition to nickel base superalloys of the type to which the presentinvention relates. Typical of other such elements are columbium andvanadium. The alloy compositions shown in Table VII are typical of thoseother alloys, including such additional elements, evaluated inconnection with the alloy of the present invention.

TABLE VIL-OTHER ALLOYS [Weight percent, balance Ni and incidentalimpurities] C Cr C0 M0 W A1 Ti A14-Tl B Other of aluminum and titaniumand alloy 7 which is above the range of the total of aluminum andtitanium, none passed the weld restraint patch test. Alloy 16, which hadbeen tested before and passed was not retested in this evaluationbecause of its lower strength. As shown in Table VI, alloy number 6formed a transverse crack on stress relief As can be seen from thestrength properties of the following Table VIII, the addition ofcolumbium and vanadium to bring the total amount of such hardeningelements at least Within and in most cases greater than the range of thepresent invention, resulted in little if any improvement in strengthproperties.

TABLE VIII [Strength properties after 2,000-2,150 F. solution and 1,400"F. age] All of the compositions of Table VII were reducible to sheetform with the exception of alloy Example 34 which could not be convertedto sheet. Alloys 27, 28, 29 and 30, which were the best of the alloysincluding columbium, were evaluated in the weld restraint patch test.Alloy 31 was not because it was too brittle: it fractured before the0.02% yield point could be reached. All of the alloys 27, 28, 29 and 30developed severe cracks in the welds during stress relief after weldingso that none of them were capable of being evaluated further forweldability.

Although some alloys of the nickel base super alloy type can tolerateiron in relatively large amounts, iron is specifically excluded, exceptperhaps as a slight impurity, in the alloy of the present inventionbecause of its tendency to reduce stress rupture properties.

Referring to the drawing, there is shown a comparison between the alloyform of Example 1 within the scope of the present invention as shown inTable I, that of another reported alloy A having a composition, byweight, of 0.05% C, 2.0% Al, 3.0% Ti, 6.0% Mo, 19.0% Cr, 12.0% Co, 1.0%W with the balance substantially nickel and a well known Alloy B havinga composition, by weight, of 0.05% C, 3.0% Al, 3.0% Ti, 4.0% Mo, 17.5%Cr, 16.5% Co with the balance essentially nickel and incidentalimpurities and including no tungsten. The superiority of the presentinvention over a wide range is shown. The graph form is theLarson-Miller parameter type reported in the Transactions of theAmerican Society of Merchanical Engineers, 1592, vol. 74, at pages765-771.

Although this invention has been described in connection with specificexamples and embodiments, it will be recognized by those skilled in themetallurgical art, the variations and modifications of which theinvention is capable. It is intended yin the appended claims to coverall such modications and variations.

What is claimed is:

1. An improved, Wrought nickel base alloy characterized by improvedweldability in sheet form along with good strength properties up toabout 1800 F., the alloy consisting essentially of, by weight:

0.07-0.17% C, said carbon percentage being greater than that requiredfor deoxidation and in addition being suicient for forming discreteagglomerated grain boundary carbides;

the sum of Al and Ti being 5.5-6.5;

with the balance Ni and incidental impurities.

2. The alloy of claim 1 in which the carbon range is 3. The alloy ofclaim 2 in which:

Co is 14-16%;

Cr is 13-15%;

Mo is 5.5-6.5%; and

W is 2.5-3.5%.

References Cited UNITED STATES PATENTS 2,809,110 10/1958 -Darmara 75-1712,975,051 3/1961 Wilson et al. 75-171 2,977,222 3/ 1961 Bieber.3,107,167 10/1963 Abkowitz et al. 3,110,587 11/1963 Gittus et al.3,155,501 11/1964 Kaufman et al. 3,385,698 5/1968 MacFarlane et al.75-171 OTHER REFERENCES Journal of Metals, February 1954, relied onpages 2li-218.

CHARLES N. LOVELL, Primary Examiner U.S. Cl. X.R. 14S-32

